Generally, in a MOS transistor configuration, positions and shapes of source, drain and gate vary according to the requirements. The MOS transistor has to maintain an appropriate resistance (Rdson) between drain and source in a turn-on state irrespective of its configuration. After the MOS transistor has been fabricated, a resistance between the source and drain is measured using a probe card while power is applied to a gate of the MOS transistor. This facilitates the measurement if the source and drain are provided at an upper side of the wafer. Yet, if the drain is provided to a bottom or otherwise backside of the wafer, a chuck is separately used to contact the drain.
FIGS. 1 to 3 illustrate an on-resistance measuring method in a backside drain wafer according to the related art. Referring to FIG. 1, a unit MOS transistor, of which backside is a drain, includes a lightly doped n-type epitaxial layer 2 formed on and/or over a heavily doped n-type substrate 1, a p-type channel layer 3 formed on and/or over the epitaxial layer 2, a gate 4 embedded in the channel layer 3 and a portion of the epitaxial layer 2, a source provided at an upper peripheral part of the gate 4 and the drain provided at a backside of the substrate 1. If power is applied to the gate 4, a channel is vertically formed to electrically connect the source 5 and the drain 6. Hence, if the transistor is turned on, a vertical current path is generated to electrically connect the source 5 and drain 6.
FIG. 2 is a diagram for configuration of an on-resistance measuring device in backside drain wafer according to a related art and FIG. 3 is an equivalent circuit diagram thereof. Referring to FIGS. 2 and 3, an on-resistance measuring device in a backside drain wafer according to a related art includes a chuck 7 which contacts the drain 6. Resistance between the source 5 and the drain 6 is measured using a probe card 8 connected to the source 5 and the gate 4 while power is applied to the gate 4. In particular, the probe card 8 and the chuck 7, which configure a measuring tool, are connected to both ends of the current path between the source 5 and the drain 6. The drain resistance is then measured using the relation between the corresponding current and voltage.
However, in case of using a metal chuck 7 directly contacting the drain 6, the intrinsic resistance of the chuck 7 and the contact resistance between the drain 6 and the chuck 7 are involved, it is difficult to perform precise measurement. To maximize reliability of the measurement value in the above measurement, the chuck 7 should be formed of a low-resistance material such as Au and the like. Yet, even if the chuck 7 is formed of Au, it is impossible to completely cut off the generation of parasitic resistance. Hence, the measured value becomes erroneous as well as a product cost of the measuring device is raised.